We had this scenario play out several times this past summer in Minnesota, I suspect mostly due to the weather. My wife and I would decide to go camping on Thursday, grab one of the last remaining sites, then show up on Friday afternoon to a half-filled park even though all of the sites were booked. I guess as long as everyone is paying upfront, I don't mind. However, if you can book 6-months out and then cancel at 3:59pm the day of, that's a little disappointing.
@LetsRide - the farther you are away from the Twin Cities in Minnesota, the more camping availability you have. I've heard that Minnesota has nice roads...